Electroacoustic and dielectric dispersion of concentrated colloidal suspensions

The determination of the permittivity of colloidal dispersions has been demonstrated to be a very useful technique to characterize the electrical state of the solid/liquid interface. This is so because of its sensitivity to such features as particle size and shape, state of aggregation, surface charge, and so on. The dielectric dispersion data are particularly suited to the evaluation of the surface characteristics when the suspension contains a high solids concentration, as in such conditions the usual electrokinetic techniques linked to optical methods are not applicable. The same advantage is shared by electroacoustic techniques, where it is a collective response of the system that matters. In this case, the experimental quantity is the dynamic (or ac) electrophoretic mobility. In this work, both methods will be used for the investigation of the electrical surface characteristics of concentrated alumina suspensions with volume fractions of solids ranging between 1 and 20%. The data will be analyzed in the frame of the so-called cell model, often used to analyze the hydrodynamic and electrical interactions between particles in concentrated suspensions. We will show that the model is capable of properly describing both their dielectric dispersion and dynamic mobility, and that the surface (zeta) electric potential calculated from electroacoustic data can be used as an input parameter to reproduce the permittivity. This will demonstrate the internal coherence and accuracy of the cell model and give clues to improve our understanding of the electrokinetic behavior of concentrated slurries